1. Technical Field of the Invention
This invention relates in general to a mobile communications system which has pagers and base stations providing two-way communication of alphanumeric messages between the pagers and the base stations. More particularly, these messages are communicated via the Global System for Mobile Communications (GSM/GPRS) network which has been set up on a world wide basis. It is specifically designed for the pager to send preset, or canned, responses to a central station which are useful in coordinating personnel responding to events, such as emergencies.
2. Description of the Prior Art
Simple paging systems are comprised of small analogue radio receivers (pagers) and one or more transmitters (base stations). The base station in these systems transmits a series of tones which represent an address code and the pagers in the system evaluate the tones. At least one pager in the network detects the tone sequence as its own code and alarms the user of the pager with a beep or vibration that the user is being called.
Paging systems have progressed over the years in both frequency and extent of usage as well as complexity, Some pagers now have subminiature radio receivers, which are capable of reproducing a voice message which has been broadcast over the network by a transmitter. Only those pagers whose address codes have been transmitted over the network will open their audio paths to receive the transmitted voice message.
Some of the more recent pagers are equipped with a numeric or alphanumeric display. These pagers are capable of displaying a message sent by transmitters using a digital code which contains the address of a specific pager, or group of pagers, and the text of the message being sent. These paging systems may use a protocol such as the Post Office Code Standardization Advisory Group (POCSAG) or Golay Sequential Code (GSC). These protocols utilize well known error detection and error correction techniques and are, therefore, tolerant to bit errors occurring during transmission, provided the errors are not too numerous in any one word.
All of the paging systems described above are strictly one-way. There is no confirmation to the sender that a message has actually reached the pager, nor does the user have any means of sending an answer via his pager.
To remedy this limitation, pagers have been developed which allow two-way communication and which have individual transmission capability. When a pager of this type is called by a base station, the pager transmits its ID code to the base station which is an affirmative response that the pager has received the call.
A conventional pager which permits two-way data communication requires more electric power than can be provided by the battery of a conventional pager. In order to solve this problem, a dedicated network of repeaters are used to send the pager's ID and message to the sending base station. A large number of repeaters are necessary for a two-way paging system to cover a large geographical area (e.g., a state or an entire country). This is a very expensive outlay needed to obtain a two-way data communication channel. In addition, a multi-repeater setup requires many different radio frequencies which are becoming more difficult to obtain from the FCC. Additional gateways are required if the communication system also accesses fax machines, the Internet or other alphanumeric devices. The dedicated two-way paging systems are ideal for a single-repeater configuration which covers a limited area (e.g., a single building or hospital).
Thus, what is needed is a two-way paging system that can be used throughout an entire country or internationally which does not require a large number of expensive repeaters. In addition, there is an increasing need to incorporate gateways to other communication networks within the paging system.
The Global System for Mobile Communications (GSM/GPRS) standard has been widely adopted in Europe and is now under consideration in several other countries outside of Europe. One of the primary goals of the GSM/GPRS standard is to enable users to move across national boundaries while still being able to communicate. Each country independently operates its own public land mobile network (PLMN) and the coverage of the PLMN is commercially confined to the borders of the country in which it is implemented. Radio coverage may overlap at national boundaries and each country may have several competing PLMNs.
GPRS (General Packet Radio Service) is a data service that allows information to be sent and received across a mobile telephone network. It supplements today's Circuit Switched Data and Short Message Service. Theoretical maximum speeds of up to 171.2 kilobits per second (kbps) are achievable with GPRS using all eight timeslots at the same time. This is about three times as fast as the data transmission speeds possible over today's fixed telecommunications networks and ten times as fast as current Circuit Switched Data services on GSM networks. By allowing information to be transmitted more quickly, immediately and efficiently across the mobile network, GPRS may be a relatively less costly mobile data service compared to SMS and Circuit Switched Data. GPRS facilitates instant connections whereby information can be sent or received immediately as the need arises, subject to radio coverage.
When a GSM/GPRS customer subscribes to the GSM/GPRS system, the subscriber normally subscribes to a single PLMN, which is commonly referred to as the home-PLMN. The subscriber's terminal, which is commonly referred to as the mobile station (MS), is typically thought of as having two principle components, namely, a first component relating to hardware and software for the radio interface and a second component relating to the subscriber identification information, which corresponds to the subscriber identity module (SIM). The SIM component can be removed from the subscriber's mobile station and is typically much smaller than a credit card. The SIM is assigned a unique identity within the GSM/GPRS system, which is commonly referred to as the international mobile subscriber identity (IMSI). The IMSI is an identifier which is internal to the GSM/GPRS system and which is utilized by the GSM/GPRS system to identify the mobile station. A second identifier, commonly referred to as the temporary mobile station identity (TMSI), is assigned to each mobile station when it enters a new registration area and is valid only within that particular registration area. These identifiers should not be confused with the telephone number assigned to the mobile station. The TMSI and IMSI identifiers are utilized by the GSM/GPRS system to track the mobile station within a registration area and as it moves from one registration area into another registration area of the GSM/GPRS network.
Currently, a stand-alone, two-way paging system which utilizes the GSM/GPRS network does not exist. The GSM/GPRS standard provides a short message service (SMS), which is suitable for alphanumeric paging SMS is now widely utilized. The SMS provides for communication of alphanumeric messages at over GSM/GPRS. However, in order for subscribers to have access to any of the GSM/GPRS services, including SMS, telephone companies require subscribers to pay cellular telephone rates. Although some companies provide cellular telephones with paging-type circuitry which can be used to transmit alphanumeric messages over the GSM/GPRS network, since GSM/GPRS subscribers are required to pay cellular telephone rates, GSM/GPRS subscribers typically communicate by voice over their cellular telephones rather than by sending alphanumeric messages over the SMS. Therefore, there has been no motivation or desire to provide stand-alone pagers for the GSM/GPRS network.
In areas in which the number of available frequency bands is very limited, the typical pagers described above and the systems in which they operate are unsuitable due to the large number of frequency bands needed for their operation. For example, in small countries, such as Switzerland, these typical paging systems are unsuitable because the number of frequency bands required for their use is not available. It would be desirable to provide a two-way paging system which utilizes the currently under-utilized SMS bandwidth and which does not require the subscriber to have a cellular telephone. One of the advantages of such a system is that it would allow government emergency response agencies, such as the fire department, to have two-way communication over relatively simple paging devices and thus would eliminate the need to purchase cellular telephones for this purpose, which generally are much more expensive than pagers.
Accordingly, a need exists for a stand-alone, two-way pager which is capable of providing two-way communication over the GSM/GPRS network.